Hydrologic Versus Hydro-geomorphic Perspectives

Scientists tend to think about floods in two different (but related) ways, one being strictly hydrologic and the other requiring an evaluation of the floodplain topography and how different flow regimes might impact the channel and surrounding areas. In module 3, we explored how hydrologic analyses (analyzing the patterns such as the frequency, duration, and magnitude of flood events) could be used to characterize the river flow regime (e.g., how often does the Susquehanna River exceed 800,000 cfs?). However, hydrologic analysis alone does not provide any information about extent or duration of flooding across the landscape (e.g., at 800,000 cfs, which parts of the natural floodplain or streets of Wilkes-Barre, Pennsylvania will be flooded?). To predict how much of the floodplain might be inundated by a given flow, we also need to consider the channel and floodplain topography (a hydro-geomorphic analysis). For example, a river system with low channel banks and a broad, flat floodplain will experience more frequent flooding of greater extent than a river system with tall banks and a narrow floodplain, given the same flow regime.

Because the size of a river channel might change over time, the relationship between the hydrologic flood frequency and hydro-geomorphic mapping of the area inundated may also change, as discussed below in the section on hydrologic non-stationarity. For example, the Minnesota River (a major tributary to the Mississippi River) has widened by nearly 50% in the past 3-4 decades. Therefore a flood that may have inundated a significant amount of floodplain 50 years ago may now be entirely conveyed within the channel itself. Thinking back to the example of the Lehigh River in Figures 4 and 5 it is very likely that the 1941 flood (the lowest on record) did not fill the channel and inundate the floodplain. 1943 and 1944 had moderately high peak flows, but may also not have gotten out of the channel because the massive 1942 flood would have widened and deepened the channel. Over the following years, the channel would likely have narrowed again, in response to relatively smaller floods. Flood frequency analysis, discussed below and in the exercise associated with this module, is strictly a hydrologic analysis. Hydro-geomorphic analysis is needed to estimate risk of flood damage. Both types of analyses may be important for engineering plans and ecological studies. High resolution topography data (elevation data with a vertical precision of about 15 cm and horizontal resolution of about 1 m, also known as 'lidar' for Light Detection and Ranging) is revolutionizing the way we make flood inundation predictions. Lidar data contains very detailed information about the ground surface, as well as vegetation on the floodplain, which exerts strong influence on the velocity and depth of the water. Many states are revising their flood risk maps using this new high resolution data.